HOW THE MEDIAN S-BAND CIRCULAR POLARIZATION RATIO OF KILOMETER-SCALE CRATERS EVOLVES WITH TIME ON THE LUNAR MARIA

Measurements from the Miniature Radio Frequency (Mini-RF) instrument provide useful data for evaluating how the surface properties of lunar craters evolve with time. In particular, S-band circular polarization ratio (CPR) measurements are sensitive to the rockiness of the uppermost meter of regolith and/or roughness of surface topography around craters. We extracted radial CPR profiles from 6,241 craters in the 800 m-2 km diameter range on the lunar maria, and used pre-existing estimates for the age of these craters to construct median profiles of 14 different age classes. Although the CPR profiles for individual craters of a given age vary significantly, in aggregate, craters show a clear and systematic evolution of CPR with time. Fresh craters (<200 Ma) have ejecta with elevated CPR out to distances of more than three crater diameters beyond their rims. The extent and magnitude of this enhanced CPR in craters’ ejecta fades relatively rapidly as craters age. Within crater interiors, the CPR signature increases for 400-600 Myr and then declines. This suggests that crater interiors initially becomes rockier or rougher at centimeter-to-decimeter as regolith develops after crater formation. Additionally, our results illustrate that crater interiors and ejecta have CPR evolution that is significantly decoupled. The CPR enhancement in crater ejecta fades faster than in crater interiors. This has potential implications for understanding so-called “anomalous” craters at the lunar poles that have raised CPR in their interior compared to their ejecta and have been used to infer the presence of water ice.